A resin material having a metal film pattern formed thereon has long been used in the field of electronics parts, decorative products and the like. For example, a film having a metal film formed as a circuit pattern on the surface of a resin substrate has widely been used as a flexible printed circuit board or the like. Conventionally, formation of a metal film pattern on the surface of a resin substrate has been carried out commonly by a process of firstly laminating the resin film substrate with a copper foil through an adhesive to produce a copper-clad laminate, drawing a desired pattern thereon, and dissolving copper foil of the part other than the pattern to remove by an etching process.
In association with the recent development of the electronics industry, however, demands for making a metal film pattern finer and for making adhesion of a metal film higher have been increased.
On the other hand, the above-mentioned conventional method has problems such that heat resistance of the adhesive bonding a resin film substrate and copper foil is low, and as a result, adhesiveness of the metal film becomes low.
According to the above-mentioned etching process, in addition, it may be difficult to etch a metal film pattern with high accuracy since it is difficult to control the rate of dissolving of the metal film.
Furthermore, it is known that the bonded interfacial surface of the metal film and resin is corroded by an etching liquid, and it might cause deterioration of adhesiveness particularly in the case of a fine pattern.
Accordingly, in a manufacturing process comprising an etching process, there has been a problem such that it is difficult to handle a fine pattern with high adhesiveness such as a fine pattern having a width of several dozen micrometers (μm).
In order to overcome the above problems, a method of forming a metal film on the surface of a resin film without using adhesives has been developed, and an evaporation method and a sputtering method thereof are used.
However, in the case of using an evaporation method, reliable adhesion is insufficient in some cases. In the case of using a sputtering method, adhesion strength is expected to be high but an etching process is required for forming a metal film pattern and there remains a problem such that it is difficult to deal with making a pattern finer.
A direct writing technology and a direct metalization technology have been attracting attention in these days.
Examples of the direct writing technology include a method of printing a pattern using nanosilver paste and baking it to form a conductive pattern and a method of drawing a pattern directly using an ink containing plating catalyst such as palladium and forming an electroless plating film on said ink.
These methods have advantages such that an etching process can be omitted by drawing a pattern directly. However, there is a problem in adhesion strength between paste or ink and a resin. In addition, paste and ink to be used are highly expensive. Furthermore, the conductive pattern obtained by these methods has high electric resistance, and therefore, electrical property as a metal film is insufficient.
The direct metalization technology is expected in its highly reliable adhesion between a metal film and a resin. According to the Patent Documents 1, 2 and 3, a polyimide resin film is treated with alkali to open its imide ring to form carboxyl groups, a metal salt is adsorbed to said carboxyl group and said metal salt is reduced to form a metal film. This method is expected in highly reliable adhesion because a metal film is formed through functional groups formed on a polyimide resin film without using adhesives. According to these methods, however, a metal film is formed on the whole surface of a substrate in advance, and subsequently an etching process must be carried out in order to make the metal film patterned, which is same as the conventional methods.
In the etching process, the metal film on the interfacial surface of the resin might be corroded and there still remains a problem that it is difficult to deal with a thin line pattern having highly reliable adhesion.
The Patent Document 4 discloses a method of cleaving an imide ring by applying an alkali solution on an inorganic thin film formation portion of a polyimide resin substrate by an ink-jet process, adsorbing a metal ion to said portion to produce a metal salt and reducing said metal salt to form an inorganic thin film pattern. According to this method, it is demonstrated that an inorganic thin film pattern can be formed on a polyimide resin substrate without using an etching process. However, since a low-viscosity solution is discharged according to the ink-jet process in general, there are problems such as bleeding and/or repelling of the alkali solution on the polyimide resin, which might cause difficulty in forming a fine pattern.
In addition, the above-mentioned patent document does not disclose a finding concerning suppression of the falling of metal ions in the metal ion reduction process. Therefore, there is a problem that it is necessary to use a highly-concentrated metal ion solution. Furthermore, it might be unable to adsorb sufficient metal ions for forming an inorganic thin film.